I took a look at their website, and since I don't have a BoA account, I couldn't try it, but it looks like it should work just fine by what they have written in their FAQ.
Of course, they're probably doing that stupid JavaScript bull...
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche
I tried Mozilla 0.9.9 and Netscape 6.2 and both would not log in, and reported Javascript errors. IE 6.0 did the same thing. But once XP was uninstalled and RRR to 98SE (IE 4.0) it worked fine.